Emissions produced by an engine shortly after start-up may be higher than after an engine is at optimal operation temperature. Higher emissions may be due to reduced fuel vaporization and atomization under certain conditions. For example, after start-up of the engine, the fuel system pressure may not yet have attained a pressure to cause sufficient atomization of the fuel within the combustion chamber, which may result in increased emissions. Furthermore, lower engine temperatures after start-up can further reduce the vaporization rate of the fuel. As yet another example, vaporization rate can vary with fuel composition. For example, blended fuels containing gasoline and alcohol may have lower vaporization rates than fuels containing only gasoline or lower concentrations of alcohol.
For example, during a cold start, where an engine is started at a temperature below its operation threshold value, fuel may not be combusted as efficiently as when the engine operates at or above an optimal operation temperature. Cold engine temperatures may also lower the exhaust temperature, allowing for the condensation of more liquids, an example of which is water, out of exhaust gases. Thus, during cold start, emissions, such as hydrocarbons in the exhaust, including uncombusted fuel and partially combusted fuel, may be released.
Depending on the fuel, the emissions may be further increased during cold start. For example, fuels containing ethanol may have relatively low volatility. Low volatility fuels may require higher fuel to air ratios for optimal combustion and may not heat up an exhaust system or an engine as quickly as other fuels, for example gasoline. In this way, low volatility fuels may produce relatively more undesired byproducts under cold start conditions than other fuels.
A catalytic converter may be used as part of an exhaust system to convert undesired byproducts into less harmful byproducts. When the catalytic converter is at or above an optimal operation threshold value, referred to as the light-off temperature, the catalytic converter may effectively reduce such undesired byproducts. However, under cold start conditions, a catalytic converter is generally below its light-off temperature and inefficient conversion of exhaust occurs.
Systems have been developed to address the conditions which occur at cold start. For example, U.S. Pat. No. 5,396,764 describes an approach for selectively filtering exhaust gasses with a breathing bellows apparatus coupled to a solid filter which may be used for storing and oxidizing exhaust. The bellows may respond to pressure changes brought about by increased temperatures in the catalytic converter when it reaches light-off temperature, opening up an interlocking vent system that allows exhaust to by-pass the filter. Another approach described in U.S. Pat. No. 6,357,227 uses a bypass in the exhaust system, so that undesired byproducts are oxidized by an aqueous reagent. The bypass system may be controlled by valves and in some examples includes multiple compartments for storing water and reactants, such as urea, to produce reagents, such as ammonia.
The inventors herein have recognized various issues related to these approaches. For example the use of breathing bellows and solid filtering systems may be subject to high amounts of wear and tear and may depend on complicated mechanical systems for selectively filtering the exhaust. Further, the use of chemical reagents may require continual addition of said chemicals to an exhaust system. Additionally, reagents, such as ammonia, may be undesirable themselves and may cause harm to the environment. Furthermore, such approaches do not specifically address the concerns related to emissions from less volatile fuels, for example ethanol.
As one approach, the inventors have recognized that at least some of the above issues may be addressed by a system adapted to trap liquid exhaust in an engine's exhaust system during cold start before a catalytic converter reaches its light-off temperature. In one example, a portion of the exhaust may be condensed to form liquid exhaust. The liquid exhaust may be stored in a trap and release delayed until after light off temperature is reached. As such, the level of wet exhaust reaching the catalytic converter prior to light off temperature being obtained may be reduced. Reducing the emissions during cold start, results in an increase in the efficiency of the exhaust system. Such systems, devices and methods may be applied to an ethanol-based fuel system.